WO2017017343A1

WO2017017343A1 - Method for determining an acceptable maximum alternating stress for a part subjected to cyclic solicitations; unit for determining such a stress

Info

Publication number: WO2017017343A1
Application number: PCT/FR2016/051853
Authority: WO
Inventors: Geoffrey Juan DESMEURE; François Pierre Michel COMTE; Anaïs Suzanne Amélie GAUBERT; Arnaud Cyril LONGUET; Marine Laëticia Camille CHEVALIER; Michael Piers HANSOM; Lucie Marie Ida LANCIAUX
Original assignee: Safran Aircraft Engines
Priority date: 2015-07-27
Filing date: 2016-07-19
Publication date: 2017-02-02
Also published as: GB201801326D0; FR3039676B1; US20180232472A1; GB2558092B; US10984153B2; GB2558092A; FR3039676A1

Abstract

Method for determining an acceptable maximum alternating stress (δ_{alt_max}) at a point (P) of a part (50) subject to cyclic solicitations (F) wherein: a) assuming that the part has an elasto-viscoplastic (EVP) behaviour, the part is simulated as undergoing constant solicitations equal to a threshold value (F_moy) during a plateau period (P2); b) on the basis of the results of the simulation, a final static stress (δ_{stat_fin}) is determined at said point (P), at the end or after the end of the plateau period; and c) for the point (P) in question, the acceptable maximum alternating stress (δ_{alt_max}) is determined using a Goodman diagram, said stress being determined for a static stress equal to the final static stress (δ_{stat_fin}), the duration (D) of the plateau period being equal to the duration of solicitation of the samples that were used to establish the Goodman diagram.

Description

A method of determining a maximum permissible alternating stress for a part subjected to cyclic stresses;

Unit for determining such a constraint DOMAIN OF THE INVENTION

The invention relates to means and methods for determining the maximum permissible alternating stress at a point in a workpiece that can be subjected to substantially cyclic stresses. Such stresses are typically the centrifugal forces experienced by the rotating parts of rotating machines, or the stresses to static parts, but arranged near rotating parts.

BACKGROUND

The invention is placed within the framework of certain modeling hypotheses, presented below. These hypotheses in themselves are known in mechanics, for the study of the dynamic behavior of the parts.

According to these hypotheses, we consider that the constraint σ that develops within a part subjected to cyclic stresses is the sum of two terms: a static stress term o _sta t, which is the part of the slow-moving constraint ; and a term of alternating stress where it _is, which is the part of the constraint to rapid changes; the latter term notably contains the cyclic or substantially cyclic variations of the stress, the frequency of which is substantially higher than that of the static constraint o _sta t-

The static stress a _sta t can therefore be considered as a smoothed value in time of the (total) stress σ.

It is therefore considered that the tensor of the static stresses and the tensor of the alternating stresses are linked by the equation:

O = Ostat + Oait

The maximum allowable alternating stress', denoted o _has it_max, for a material subject to cyclical stresses, is the maximum alternating stress that can support the material when subjected to a predetermined number of stress cycles. This constraint maximum permissible alternating a _a it_max is defined at a given instant depending on the static constraint _s condition suffered by the material at this time, the value of the static constraint _sta t resulting in turn different stresses applied to the workpiece.

Usually, the maximum permissible alternating stress _a it_max relative to a material is determined by performing cyclic stress tests on specimens made of this material. These tests are performed at a temperature and for a predetermined number of cycles. This predetermined temperature usually corresponds to the temperature at which the material is carried in operation; the number of cycles is defined according to the number of stress cycles to which the material is subjected during the expected life of the part of which it is part.

The term "part" here denotes either an integrally formed part or a set of separate parts connected together. It here mainly designates rigid (or substantially rigid) parts and excludes articulated assemblies and fluids.

The cyclic stress tests indicated above are carried out for different solicitation values and make it possible to establish a Goodman diagram of the material, in a manner known per se.

A Goodman diagram is a diagram representing on the abscissa, the static stresses undergone by the material, and on the ordinate, the alternating stresses undergone by this one. This diagram represents, as a function of the static stress undergone by the material, the maximum admissible permissible stress which can be experienced by it without it breaking when it is subjected to the number of predetermined cycles of stress (by example, 10 ⁷ cycles) being brought to the also predetermined temperature.

Also, to determine if a part, defined in particular by its geometry (its three-dimensional model) and its material, is able to withstand the mechanical stresses that it is expected to undergo, it usually proceeds as follows:

(a) the behavior of the part is simulated numerically for a period of time during which it is subject to envisaged solicitations; the simulation is performed taking into account for the material a model of elastic behavior;

b) determining at the end of the simulation the static stress and the alternating stress undergone at the point of the part considered;

c) for this value of the static stress, it is determined using the Goodman diagram (established by the method described above) if the alternating stress in the material at the point considered is less than the maximum permissible alternating stress.

However, it has been found especially in the development of parts exposed to high temperatures, and in particular subject to relatively high frequency stresses (more than 1000 Hz), for example aeronautical turbine engine high pressure turbine blades, which the previous method does not give satisfactory results. This method would indeed lead to prohibiting the use of certain materials for certain applications, or certain part geometries, whereas in practice, experience shows that these materials and / or these part geometries can be chosen for the considered applications. .

This phenomenon can be better understood thanks to Figure 1.

This figure shows the Goodman diagram associated with a material (curve 10), in comparison with a theoretical curve corresponding to the real behavior of the material (curve 12).

In this figure, the abscissa axis indicates the static stress Gstat in the material at the point considered, and the ordinate axis indicates the alternating stress a _a it at this point.

The curve 10 is the curve of the Goodman diagram representing the behavior of the material considered, at a given temperature T, and for a predetermined number of cycles N (in this case, 10 ⁷ cycles).

The points of curve 10 are the points for which the breaking of the material occurs just after the number of cycles N, namely 10 ⁷ cycles. Therefore, the points of the diagram that lie under the curve 10 have static stress values a _s state and alternately constraint _has it for which breakage of the material occurs for a number of cycles greater than 10 ^7, so that conversely, for the points of the diagram which lie above the curve 10 have static stress values has _sta t and alternating stress o _has it for which the fracture of the material occurs for a number of cycles lower than 10 ^7.

The curve 10 is a curve drawn by performing cyclic tensile tests on test pieces formed by the considered material, brought to a temperature T, for different stress values under static o _s tat-

Because of the limitations of tensile testing machines, which in practice can not operate at frequencies above 100 Hz, Goodman's diagrams are representative of the behavior of the material mainly when the stresses applied to the material vary at a frequency also less than or equal to 100Hz.

As can be understood, these conditions are too restrictive and often do not represent the frequencies of the solicitations actually applied to the parts, which can be much higher.

According to the Goodman diagram, it should not be possible to exploit parts in which the static stress undergone by the material is greater than a value o _max represented in FIG.

However, it has been found in practice that many parts support such constraints.

It can be deduced that the curve that should be plotted to represent the actual behavior of the material is not the curve but a curve of the type of the curve 12, shifted with respect to the curve 10.

Therefore, a Goodman diagram such as the one presented above, and the method of checking the suitability of a part to undergo certain stresses, as defined above by steps a) to c), can not not always be used to realistically assess whether a part, defined in particular by its geometry and its material, is able to undergo a number of predetermined cyclic stresses without breaking.

PRESENTATION OF THE INVENTION

The objective of the invention is therefore to overcome the shortcomings presented above. For this purpose, according to the invention there is provided a method for determining a maximum permissible alternating stress at a point in a workpiece that can be subjected to substantially cyclic stresses, which provides a value of the maximum permissible alternating stress. for the point of the room considered.

The maximum allowable alternating stress values thus determined may be used to verify that the alternating stress does not exceed the maximum permissible alternating stress at any point in the room considered.

This result is achieved by virtue of the fact that the method comprises the following steps:

a) numerically simulates that the part, during a period of plateau, undergoes constant stresses equal to a threshold value;

the simulation taking into account an elasto-viscoplastic behavior model (EVP) for a material constituting the part at the point considered;

b) from the results of the simulation, determining a final static stress at said point, at the end or after the end of the plateau period; and

c) for the point of the part under consideration, is determined using a first Goodman diagram the maximum allowable alternating stress has _a it_max, the latter being determined for a static load equal to the end stress plateau has _sta t_fin;

the duration of the bearing period being substantially equal to the duration of stressing of the test pieces used to establish said first Goodman diagram.

In the method as defined above, the various constraints mentioned (for example static stress, etc.), are either tensors or scalar quantities (real numbers); the nature of the constraint considered must be determined according to the context.

In particular during step b), the final static stress that digital simulation provides is usually a tensor.

From this value of the tensor, the final static stress σ ^ _ _ηη is computed for the moment at the end of the plateau: this is calculated in scalar form. This value _ _Πη (scalar) that is used in step c) to determine the maximum allowable alternating stress o _has it_max-

The scalar value of the constraint can be calculated in different ways depending on the stress tensor in the part. Usually, the stress of Von Mises is chosen as the scalar constraint; however, while remaining within the scope of the invention, the value of the stress in scalar form can be defined differently from the stress tensor. It can for example be a maximum principal constraint, or other.

Advantageously, the invention is applicable to parts subjected to stresses of any kind: mechanical, thermal, or other.

The solicitations may comprise point, linear and / or surface forces.

Stresses in the form of a force field or comparable to a force field, such as centrifugal forces, can of course also be taken into account as solicitations within the scope of the invention.

Solicitations can also be defined as boundary conditions, such as boundary conditions of position, velocity, and / or acceleration.

It will be understood that in the method defined above, the simulation step a) may comprise not only the period of bearing, but also other periods during which the behavior of the part is simulated, these periods being able to extend before and or after the period of landing.

Thus, in a preferred embodiment, in step a), it is simulated numerically that the part, before the period of bearing, undergoes stresses varying from a zero value to the threshold value, during a rising period. in charge. Taking into account the simulation of this ramp-up period makes it possible to more realistically represent the rise in the stresses in the part under the effect of the stresses applied.

An important feature of the process according to the invention defined above is that it takes into account, to represent the behavior of the material, a model of elasto-viscoplastic behavior (EVP).

The simulation carried out taking into account, for the material, "a model of elasto-viscoplastic behavior (EVP)", is a simulation of material in which:

. the deformation ε of the material is the sum of a term expressing its elastic deformation ε _ε and a term expressing its visco-plastic strain ε _ρ ; and

. the stress σ experienced by the material is a function notably of the plastic deformation rate of the material of _p / dt.

In the context of the invention, any model of elasto-viscoplastic behavior (EVP) can be used.

Step a) of numerical simulation of the behavior of the part can be done by taking into account the elasto-viscoplastic behavior (EVP) of the material either during the iterative calculation steps, or in post-processing.

In the first case, during the simulation step a), at each point of the material at least in the studied zone of the part, the deformation ε indicated above is equal to the sum of an elastic deformation term e _e and a plastic deformation term ε _ρ .

Advantageously during the implementation of the invention, it is not necessary to make the hypothesis of an elasto-viscoplastic behavior of the material for the whole of the piece: It is sufficient to make this assumption for the part of the piece that one wishes to study precisely; for the rest of the room, different, simpler behavioral assumptions can be used, for example an elastic behavior model for the material. This last hypothesis can for example be retained for parts of the room that are far from critical areas.

In the second case (taking into account the elasto-visco-plastic behavior of the material in post-processing), the simulation comprises two steps:

al) a 'elastic' deformation z _e of the material is calculated by elastic type calculation; then a2) the deformation ε of the material is determined by summing the gross strain ε _ε calculated previously and a corrective term ε _ρ . This corrective term reflects the visco-plastic deformation ε _ρ of the material; it can for example be determined on the basis of local energy equivalence or following a scaling approach.

In this second case, at the end of the second calculation step, an estimate of the total strain which comprises a plastic or visco-plastic part is obtained.

The method is particularly useful for determining the maximum permissible alternating stress for parts subjected to elevated temperatures, that is to say, more specifically, temperatures that cause significant material damage by creep. These temperatures may be for example temperatures above 750 ° C, for which the traditional method presented in the introduction is particularly imprecise.

Advantageously, because of the material behavior model used, the method according to the invention provides satisfactory results over a wide range of R load ratios.

With respect to load, the following parameter R is designated:

R = (Ostat "O _a it) / (Ostat + O _a | _t )

When the load ratio R is low, the static stress o _s tat is low; the alternating stress eventually has a preponderant importance; the phenomenon of fatigue has a preponderant role especially with respect to the phenomenon of creep.

Conversely, when the load ratio R is high, the static stress o _s tat is high; the alternating stress then, on the contrary, normally has a minor importance; the phenomenon of creep has a predominant role especially with respect to the phenomenon of fatigue.

Advantageously, the use of an elasto-viscoplastic model makes it possible to take into account these two simultaneous phenomena (creep and fatigue), in particular by taking account of elastic deformations ε _β and visco-plastic deformations ε _ρ ; it also allows to take into account the dynamics of the loading applied to the material because the Plastic strain rate is involved in the calculation of the stress experienced by the material.

More particularly, the phenomenon of creep in the material is taken into account by the fact that it is simulated that one applies to the material stresses having a bearing, for a certain period during which the stresses are stabilized at the threshold value.

Indeed, it turned out that, although the actual stresses applied to the part are cyclic, a simplified simulation, taking into account not cyclic solicitations, but solicitations with a bearing, provides results with respect to the static stress Ostat close to those obtained when taking into account cyclic solicitations, and therefore exploitable.

For this purpose, as regards the duration of the step, it is appropriate to choose a duration substantially equal to the duration of stress of the test pieces used to draw the Goodman diagram used in step c). By "substantially equal" is meant here a step duration equal to the duration of the Goodman diagram +/- 30%, and preferably equal to the duration of the Goodman diagram +/- 15%. The duration of the simulated stage is therefore unrelated to the actual operating time of the part or service life.

The Goodman diagram is established by applying a number N of cycles fixed to the part, at a predetermined frequency f. The duration of the Goodman diagram is equal to the time that was necessary to achieve the expected number N of cycles, and is equal to the ratio N / f between this number of cycles N and the frequency f.

The Goodman chart should preferably be obtained or estimated for the same temperature as that of the material during the simulation (step a).

Advantageously, in the method according to the invention, the stresses applied to the part during the digital simulation step (step a) are particularly simple, since they are constant during the period of plateau. In particular, they are much simpler than the cyclic constraints actually applied to the material.

Advantageously, the method according to the invention can be implemented in a particularly short calculation time, contrary to other simulation methods in which the application of alternating solicitations to the part should be simulated.

Advantageously, the method according to the invention is independent of the frequency and / or amplitude of the stresses applied to the part.

As a result, the method overcomes problems encountered in the prior methods.

The stresses applied to the material during the simulation (step a) may include a ramp-up period before the dwell period.

This ramp-up period is a period during which the (simulated) solicitations applied to the part evolve from a zero value to the threshold value.

In the context of the invention, during the load increase, the stresses can either vary increasingly (at no time, these stresses do not decrease), or conversely in a non-monotonic manner, that is to say that the rise in charge then includes at least one period of growth of the solicitations and at least one period of decrease of the requests. The increase in load can include, for example, several alternating phases of growth and decrease of the stresses applied to the part. These alternate phases are for example to reproduce the stresses applied to the part during its break-in.

In one embodiment, the final static stress determined in step b) is the stress at the point considered at the end of the dwell period (In this case, there is no need to simulate the behavior of the room after the period of landing).

Conversely, in another embodiment, in step a), it is numerically simulated that the part, after the bearing period, undergoes variable stresses during a final period after the period of plateau. The final static stress determined in step b) is then a 'final end-of-period' constraint, determined at the point considered at the end of said final period.

In the case where the evaluated parts are aircraft turbomachine blades, the final period, which follows the bearing period and during which the part is subjected to variable stresses depending on the time, can correspond for example to a phase of descent of the aircraft at the end of flight.

In one embodiment, in step c), the maximum permissible alternating stress is determined using a Goodman diagram obtained by correlation or interpolation between at least two Goodman diagrams. These latter Goodman diagrams can be in particular goodman diagrams obtained directly by carrying out tests on specimens.

The method according to the invention is particularly suitable for determining the maximum permissible alternating stress for a rotating machine part, in particular a part of a rotor, such as a blade. In particular, when the part is a part of an aircraft engine, in an implementation mode during step a) the threshold value of the stresses applied during the bearing period is equal to an average value of stresses suffered by the part during a take-off of the aircraft.

The method according to the invention is particularly suitable for determining the maximum permissible alternating stress for a piece made of a metal alloy, especially a titanium or nickel alloy.

Advantageously, the invention can be used to qualify parts. It then makes it possible to determine the ability of these parts to undergo the cyclic stresses to which the parts are expected to be exposed.

For this purpose, the invention also relates to a method for verifying the ability of a part to be subjected to substantially cyclic stresses, the method comprising the following steps:

A. a maximum permissible alternating stress is determined at at least one point of the part when the latter is subjected to said stresses, by means of the maximum permissible alternating stress determination method described above;

B. at least one point in the part is evaluated for the alternating stress experienced at said at least one point;

C. for each of said at least one point, that the alternating stress is less than the maximum alternating stress permissible at the point considered and it is concluded that the part is not suitable to be subjected to said requests if the verification is negative for at least one point.

Steps A and B of this process can of course be performed in any order or in parallel.

Stage B is generally carried out by performing real tests of the part.

The verification indicated above is not made at any point in the room. It is usually done in a limited number of points, called checkpoints. Optionally, when the part is modeled by finite elements, all the vertices of the finite elements representing the part can be chosen as control points.

In a particular embodiment, the various steps of the method for determining a maximum permissible alternating stress or the process for verifying the suitability of a part according to the invention are determined by instructions from computer programs.

Consequently, the invention also relates to a computer program on an information medium, this program being capable of being implemented in a computer, this program comprising program code instructions for the execution of the steps of FIG. a method of determining a maximum permissible alternating stress or a method of checking the suitability of a part as described above, when the program is executed on a computer.

This program can use any programming language, and be in the form of source code, object code, or intermediate code between source code and object code, such as in a partially compiled form, or in any other form desirable shape.

The invention also relates to a computer readable information medium, and comprising the instructions of a computer program as mentioned above.

The information carrier may be any entity or device capable of storing the program. For example, the medium may comprise storage means, such as a ROM, for example a CD ROM or a microelectronic circuit ROM, or a magnetic recording means, for example a floppy disk or a hard disk.

On the other hand, the information carrier is generally a nonvolatile information carrier. However, the program according to the invention can be downloaded in particular on an Internet type network. Alternatively, the information carrier may be an integrated circuit in which the program is incorporated, the circuit being adapted to execute or to be used in the execution of the method in question.

The invention also relates to a unit for determining a maximum permissible alternating stress at a point in a part that can be subjected to substantially cyclic stresses, comprising:

a simulation module, configured to digitally simulate that the part, during a period of plateau, undergoes constant stresses equal to a threshold value;

the simulation module taking into account an elasto-viscoplastic behavior model for a material constituting the part at the point considered;

a static stress determination module, configured to determine a static stress at said point of the part at the end or after the end of the bearing period; and

a maximum allowable alternating stress determination module, configured to determine, using a first Goodman diagram, the maximum permissible alternating stress, this being determined for a static stress equal to the final static stress;

Finally, the invention also relates to a system for qualifying a part that can be subjected to substantially cyclic mechanical stresses, comprising:

A unit for determining a maximum permissible alternating stress as defined above, configured to determine a maximum permissible alternating stress at least at one point of the part when the latter is subjected to said requests;

An alternating stress acquisition unit, configured to acquire the alternating stress at said at least one point;

- A control unit, configured to check for each of said at least one point that the alternating stress is less than the maximum permissible alternating stress at the point considered and indicate that the part is not qualified if the check is negative for at minus one point.

BRIEF DESCRIPTION OF THE DRAWINGS

The invention will be better understood and its advantages will appear better on reading the detailed description which follows, of embodiments shown by way of non-limiting examples. The description refers to the accompanying drawings, in which:

FIG. 1, already described, is a schematic view based on a traditional Goodman diagram, on which is superimposed a curve corresponding to an effective Goodman diagram;

FIG. 2 is a diagrammatic view of a turbomachine, some blades of which have been checked according to an embodiment of the process for checking the aptitude of parts according to the invention;

FIG. 3 is a schematic view showing stresses applied to a part, in particular cyclic stresses, during a simulation.

FIG. 4 is a schematic view showing the variations of the stress in the part, when subjected to the stresses presented in FIG. 3;

FIG. 5 is a schematic view showing stresses applied to a part, and having a bearing, during a simulation carried out in an implementation mode of the method according to the invention;

FIG. 6 is a schematic view showing the variations of the stress in the part, when subjected to the stresses presented in FIG. 5;

FIG. 7 is a flow diagram of a maximum permissible alternating stress determination method according to the invention; FIG. 8 is a suitable Goodman diagram illustrating the final step of the method of FIG. 7;

FIG. 9 is a flowchart of a process for checking the aptitude of a part according to the invention;

FIG. 10 is a schematic material representation of a system for verifying the suitability of a part according to the invention, including a unit for determining a maximum permissible alternating stress according to the invention;

FIG. 11 is a functional schematic representation of a unit for determining a maximum permissible alternating stress according to the invention; and

FIG. 12 is a functional schematic representation of a system for checking the aptitude of a part according to the invention. DETAILED DESCRIPTION OF THE INVENTION

The invention will now be presented in the context of the design of a blade 50 of a moving impeller 60 of the high-pressure turbine 65 of a turbomachine 70 such as that shown in FIG.

The stresses to which the blade 50 is exposed during its operation (that is to say throughout the life of the turbomachine 70) are shown in FIG.

This figure presents the variations of the mechanical stresses F undergone by the dawn, as a function of the time t (in seconds). These stresses corresponding to a force F (in Newton) exerted mainly, but not only, along the longitudinal direction of the blade. This force F is due in particular to the centrifugal forces experienced by the blade due to the rotation of the impeller; it represents the stresses experienced by the part during the 'mission' or the operating phase during which the part is used, which can be for example, for an aircraft engine part, a take-off phase of the aircraft, etc. .

As can be seen in FIG. 3, these stresses, except for a period of increase in load PI extending from instant t0 to instant t1, are cyclic stresses which vary around an average value F _m oy FIG. 4 shows the variations of stress at a point P of the blade when it is subjected to the stresses presented in FIG. 3. FIG. 4 is represented on a time scale and with proportions quite different from Figure 3.

FIG. 4 comprises a first curve 20 representing the variations of the stress (total stress) σ at point P, as a function of time t. It also comprises a second curve 22, which represents the variations as a function of time of the static stress Ostat. Curve 22 represents the variations of the average value of the stress σ (thus for each cycle, curve 22 passes through the point having for value the mean value of the stress during this cycle). Curve 22 therefore does not take into account the high frequency variations of the constraint o. Two examples of implementation of the maximum permissible alternating stress determination method according to the invention will now be presented in connection with FIGS. 5 to 8 and in particular FIG. 7.

While FIG. 3 presents solicitation values F conventionally used to determine the maximum permissible alternating stress, in contrast FIGS. 5 and 6 represent stress and stress values used in accordance with the invention.

Preparation of the simulation

The first step a) of the method, simulation step, requires the preparation of the following data:

- digital model of dawn 50;

- model of behavior of the material of the dawn;

- parameters of the material of the dawn; and

- solicitations applied at dawn.

We first define the numerical model of dawn. This model is prepared in a format allowing the simulation stage to be carried out.

The numerical model of the dawn is usually a three-dimensional model defined by finite elements. In the context of the invention, any modeling of the part may be used: for example points connected by springs, or more generally any method of numerical modeling capable of allowing a simulation of the static, quasi-static, vibratory or dynamic behavior of the piece by computer.

The parameters relating to the material of the dawn are then determined:

The model of behavior of the material of the blade is determined, and the parameters of the material of the blade are determined.

In the context of the invention, the model of behavior of the material of the blade is a visco-elastoplastic behavior (EVP).

For example, the behavior pattern of the blade material may be a visco-plastic flow in the form of a Norton potential.

The stresses applied at dawn are also determined.

In the two modes of implementation presented here, we first define the stresses that will be applied to the dawn during the simulation step a), using a diagram such as that shown in FIG. 5.

This diagram shows the variations of the mechanical stresses F undergone by the blade (as an example of a part), as a function of time t.

This diagram includes a period PI rise in load between a time t0 and tl, a period P2 of step of the moment tl at a time t2, and a final period of the instant t2 at a time t3.

At time t0, the bias (or applied force) F is zero, as well as the deformation or displacement of the blade.

From time t1 and during the entire period P2 bearing, the bias is constant and equal to a threshold value. The threshold value is chosen generally equal to the average value Fmoy of the solicitation applied at dawn; this is what is represented in FIG. 5. This average value represents the average value of the requests during the critical phase of the mission carried out by the part; this average value is obtained by filtering or excluding high frequency (vibratory) stresses.

The value of the bias during the step P2 can also be chosen equal to the value of the maximum stress applied to the dawn, or other. The duration of the ramp-up period PI is generally negligible, e.g., less than l / ^10th of the length of the period P2 bearing. In the case of a blade such as blade 50, however, the duration of the period P1 is preferably at least one second. The duration of the P2 plateau period will be discussed below.

The final period P3 is a period representing a final stage of exploitation of the dawn, for example a descending flight phase.

Although in the example presented, the stresses F applied to the dawn are mechanical stresses, the invention is also applicable to solicitations of other nature, for example thermal, etc. s) Simulation of dawn operation

After having prepared all the elements necessary for the simulation, the simulation of the behavior of the blade is carried out.

This simulation consists in simulating that the solicitations defined above are applied to the dawn, the dawn being made of the specified material, the material of the dawn reacting according to the chosen behavior pattern.

Advantageously, since the variations of the solicitations are fairly simple, particularly during the period of plateau, the discretization of the time chosen for the simulation may comprise a number of time steps that are quite small.

For example, it is possible to have time steps of increasing duration: a very short first time step, corresponding to the ramp-up period, of duration t1 = 2 seconds;

followed by five time steps during the plateau period, exponentially increasing durations, for example successive durations equal to 10, 100, 1000, 10,000 and 100,000 seconds, respectively.

The duration of the plateau period t2 is chosen as follows:

The frequency f of the load cycles of the machine used to make the Goodman chart is generally known in advance.

Furthermore, the number N of cycles of solicitations during which the blade must be able to be exploited is known (this number N is usually the number of cycles retained for the Goodman diagrams established for the part).

The duration D of solicitation of the parts used to establish the Goodman diagrams for the material of the blade is therefore:

D = N / f

According to the invention, during the simulation, it is therefore required that the duration t2 of the plateau period be equal to the value D defined above. The bearing period therefore has the same duration as the duration of the solicitation of parts used to establish the Goodman diagram (s) of the material.

Moreover, as indicated above, FIGS. 5 and 6 correspond to two different modes of implementation.

In the first mode, the simulation can be interrupted at the end of the P2 plateau period, while in the second mode, the simulation of the three periods PI, P2 and P3 is necessary.

The reason for this difference will now be explained. b Determination of the final static stress

In the first mode of implementation, the final static stress (o _s tat_fin) is chosen for each point P as the end-of-bearing stress, that is the stress at time t2, at the end of the period of landing. It is noted here o _s tat_fin_i-

By contrast, in the second implementation mode the final static stress (o _s tat_fin) is chosen for each point P as the constraint at time t3, at the end of the final period P3. It is noted here

0 ^" statjin_2- c) Determination of the maximum permissible alternating stress

In parallel, regardless of the simulation of the behavior of the blade and the determination of the final static stress o _s tatjin, carried out in steps a) and b), a goodman diagram is established or at least obtained for the material of the piece, brought to the temperature T at which the point of the piece is worn during the operating conditions (ie solicitations) of the latter envisaged.

In the absence of a goodman diagram at temperature T, a goodman diagram at temperature T can be achieved by interpolating Goodman diagrams made for other temperatures.

The Goodman diagram is made for a number of load cycles provided for the part, namely in this case 10 ⁷ .

The maximum permissible alternating stress _at it_max, for the considered point P of the blade, is then determined simply by choosing the ordinate of the point of the curve whose abscissa is the static stress at the end of the bearing period.

Depending on the mode of implementation chosen, naturally two separate values are obtained for the maximum permissible alternating stress, a _a _{t t} _max_i and a _a it_max_2 (FIG.

Indeed, in this example, as seen in Figure 6, the stress experienced by the blade at time t2 is greater than that it undergoes at time t3; this results in inequality: G _sta t_fin_i> a _s tatjin_2.

As the curve of Goodman's diagram is decreasing

(Fig.8), it follows that conversely: o _has it_ _my x_i <a _a i _t _max_2.

This result reflects the fact that the maximum permissible alternating stress obtained by the method is the maximum permissible alternating stress at the moment for which the final static stress is chosen: as during the final step P3, the static stress decreases, consequently inversely the maximum permissible alterna- tive stress a _has it_max increases.

The method of determining the maximum permissible alternating stress _at it_max serves to design parts subjected to cyclic stresses.

An exemplary process for verifying the suitability of parts undergoing such stresses will now be described with reference to FIG. 9.

According to this method, the verification of suitability for the use of the part imposes to verify that during the exploitation of the part, the constraint undergone in all points (or rather, in a number of control points) the part remains at a value or in an acceptable range.

For this purpose, the parts aptitude verification method comprises the following steps:

We first design or choose the part geometry (Step 0, Figure 9), defined in the form of a digital model. This digital model is defined in particular by a number of points P, which are the vertices of cobblestones (or polyhedra) defining the three-dimensional shape of the part.

Specify the material of the part, and choose a model of behavior that is considered representative for this material.

Lastly, the conditions of use or exploitation of the part are specified. These conditions include the definition of all the solicitations that will be applied to the part, whether mechanical stress, thermal stress, geometric boundary conditions, etc.

These stresses are defined as a function of time and, at least during the period of plateau, have cyclic variations.

The purpose of the method will be to determine whether, for the specified material, with the specified part geometry, for different selected points of the part (called control points, the stresses experienced by the part during the intended operation (subject at the specified demands) will remain at acceptable values, for which purpose we proceed as follows:

A. In each of the control points P, the maximum permissible alternating stress is determined _at it_max when the part is subjected to the expected stresses, by the method presented above.

B. The alternating stress actually experienced for each of the points P is then evaluated. This evaluation is generally made by subjecting the part to real tests, and by measuring the alternating stress undergone by the part, by strain gages or equivalent. It is therefore denoted a it_essai- This _has alternating stress has suffered _a it_essai can especially be alternating force developed stabilized operating conditions P of the room. It can also be a maximum value of the alternating stress, for example the alternating stress at point P during a take-off phase, in the case of aircraft engine or helicopter blades.

C. It is checked for each of the control points P o that _has undergone alternating stress it_essai is less than the maximum allowable alternating stress o _has it_max at the point considered.

If during step C, for at least one point P, it is realized that the alternating stress undergone is greater than the maximum permissible alternating stress, it is concluded that the geometry provided for the part, with the material provided, does not does not allow to obtain a blade suitable for the intended use: it is then necessary to return to step 0 of the method, and modify one of the parameters of the blade, such as its geometry, the choice of material, Or other.

The method of determining the maximum permissible alternating stress presented in connection with FIG. 7 is implemented by a unit 80 for determining the maximum permissible alternating stress.

For the implementation of this method, the unit 80 comprises different modules (FIG. 10):

a simulation module 82 makes it possible to perform the simulation step a);

a final static stress determination module 84, which receives from the module 82 the simulation results; on the basis of these results, the module 84 makes it possible to determine the final static stress according to step b) for the control points P chosen for the part.

a maximum permissible alternating stress determination module 86, which receives the final static stress of the module 84. The module 86 then makes it possible to perform step c) and to determine the maximum permissible alternating stress for the different control points P.

The unit 80 for determining the maximum permissible alternating stress is itself part of a wider functional unit, constituting a system 100 for checking the suitability of a part to be subjected to cyclic stresses.

The system 100 serves to implement the method of checking the ability of a part to be subjected to substantially cyclic stresses according to the invention.

For the implementation of this method, the system 100 comprises different units (FIG. 11): Unit 80, previously described. It is used to determine the maximum permissible alternating stress for each of the control points P.

An alternating stress acquisition unit 90. The unit 90 is configured to acquire the alternating stress _a it_test at the various control points P. The value of the alternating stress can be either produced by calculation or acquired by the unit 90, the value being generated outside the system 100.

A control unit 95. From the maximum permissible alternating stress _at it_max received from the unit 80 and from the alternating stress _a it_essai received from the unit 90, the unit 95 checks for each of the control points. P the alternating stress a _has it_essai is less than the maximum permissible alternating stress a _a i _t _ _m ax- If for at least one of the control points P, the result of this check is negative, the control unit 95 sends an information indicating that the part is not fit to be subjected to the solicitations envisaged.

The functional modules for simulation 82, static stress determination 84, and maximum permissible alternating stress determination 86 previously described in the context of the maximum permissible alternating stress determining unit are software modules implemented within a computer. 100.

The computer 100 therefore constitutes a maximum permissible alternating stress determination unit within the meaning of the invention.

In addition, the evaluation unit 90 and the control unit 95 previously described are software modules also implemented within the computer 100.

The computer 100 is therefore a system for assisting in checking the suitability of a part to be subjected to stresses within the meaning of the invention.

The computer 100 presents the hardware architecture illustrated schematically in FIG.

It comprises in particular a processor 4, a RAM 5, a read-only memory 6, a non-volatile flash memory 7, as well as communication means 8 with the user of the computer. These hardware elements are eventually shared with other functional units. The read-only memory 6 of the unit 100 constitutes an information carrier according to the invention, readable by the processor 4 and on which is recorded a computer program according to the invention, comprising instructions for executing the instructions. steps of a part design process according to the invention. This program comprises in particular instructions for carrying out the steps of a method for the determination of maximum permissible alternating stress in accordance with the invention.

Although the present invention has been described with reference to specific exemplary embodiments, it is obvious that various modifications and changes can be made to these examples without departing from the general scope of the invention as defined by the claims. The invention can for example be implemented both for the design of aerospace turbomachines as for that of terrestrial turbines or engines ... Therefore, the description and the drawings should be considered in an illustrative rather than restrictive sense.

Claims

1. A method of determining a maximum allowable alternating stress (o _has it_max) at a point (P) of a workpiece (50) capable of being subjected to stresses (F) substantially cyclic, the method comprising the steps of :

a) it is numerically simulated that the part, during a plateau period (P2), undergoes constant stresses equal to a threshold value (F _moy );

b) from the results of the simulation, determining a final static stress (o _sta t_fin) at said point (P), at the end or after the end of the plateau period; and

c) for the point (P) considered, using a first Goodman diagram, the maximum permissible alternating stress (o ^" ait_max) is determined, this being determined for a static stress equal to the final static stress ( o _s tatjin);

the duration (D) of the bearing period being substantially equal to the duration of stressing of the specimens used to establish said first Goodman diagram.

2. Determination method according to claim 1, wherein said first Goodman diagram is obtained by interpolation between at least two Goodman diagrams.

3. Determination method according to claim 1 or 2, wherein the part is a part (50) of a rotating machine (70), in particular a part of a rotor (60), such as a blade.

4. Determination method according to any one of claims 1 to 3, wherein the part is a part of an engine (70) aircraft, and in step a), the threshold value (F _moy ) solicitations applied during the bearing period is equal to an average value of stresses on the part (50) during a take-off of the aircraft.

The method of determining according to any one of claims 1 to 4, wherein

in step a), the part is simulated numerically after the bearing period undergoes variable stresses during a final period (P3) after the bearing period (P2); and

the final static stress (a _sta t_fin) determined in step b) is a final end-of-period constraint (o _s tat_fin2) at said point (P) at the end of said final period (P3).

The determination method according to any one of claims 1 to 5, wherein

in step a), it is simulated numerically that the part, before the period of bearing, undergoes stresses (F) varying from a zero value to the threshold value (F _moy ), during a period (PI) of scalability.

A method of checking the ability of a workpiece (50) to be subjected to substantially cyclic stresses (F), the method comprising the steps of:

A. a maximum permissible alternating stress (o ^" a_max) is determined at at least one point (P) of the workpiece when it is subjected to said stresses, by means of the method according to any one of claims 1 to 6;

B. evaluating said at least one point (P) of the workpiece the alternating stress (o _has it_essai) realized in said at least one point;

C. it is checked for each of said one or at least one point that the alternating stress (o _has it_essai) is less than the maximum allowable alternating stress (o _has it_max) at the point considered and it is concluded that the workpiece is not capable of be subject to such solicitations if the verification is negative for at least one point.

A computer program on an information carrier (6), operable in a computer (100), having program code instructions for performing the steps of the method according to any one of: Claims 1 to 7, when said program is run on a computer.

Computer-readable information carrier (6), comprising the instructions of a computer program according to claim 8.

10. Unit (80) for determining a maximum allowable alternating stress (o _has it_max) at a point (P) of a workpiece can be subjected to stresses (F) substantially cyclic, comprising:

a simulation module (82), configured to digitally simulate that the part, during a plateau period (P2), undergoes constant stresses equal to a threshold value (F _moy );

the simulation module taking into account an elasto-viscoplastic behavior model (EVP) for a material constituting the part at the point considered;

a final static stress determining module (84), configured to determine a final static stress (o _s tat_fin) at said point of the part at the end or after the end of the bearing period; and

- a module for determining maximum allowable alternating stress (86), configured to determine using a first Goodman diagram the maximum allowable alternating stress (o _has it_max), the latter being determined for a static stress equal to the final static stress (o _st at_fin);

A system (100) for assuring the ability of a workpiece (50) to be subjected to substantially cyclic stresses (F), the system comprising:

- A unit (80) for determining a maximum permissible alternating stress according to claim 10, configured to determine a maximum permissible alternating stress (a _a it_max) at least at a point (P) of the part when it is subjected to said requests;

An alternating stress acquisition unit (90), configured to acquire the alternating stress at said at least one point;

- A control unit (95) configured to check for each of said one or at least one point (P) that the alternating stress (a _has it) is less than the maximum allowable alternating stress (o _has it_max) at the point considered and indicate that the part is not qualified if the verification is negative for at least one point.